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Diagnostic & Troubleshooting Flashcards

1
Q

What are troubleshooting steps?

A
  1. Gather information & symptoms
  2. Isolate the problem
  3. Implement a solution
  4. Problem solved? If not, repeat steps 2 & 3
  5. Document the steps you took for future reference
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2
Q

How can you gather information?

A
  1. Question end-users
  2. Protocol analyzers (Wireshark, etc)
  3. Physical troubleshooting tools (cable tester, others)
  4. Network management software
  5. CMD network commands
  6. show and debug commands
  7. IP SLA: monitors network performances in real-time
  8. Neighbor Discovery Protocols
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3
Q

What are the OSI troubleshooting methods?

A
  1. Bottom-Up
  2. Top-Down
  3. Divide & Conquer
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4
Q

What is Bottom-Up troubleshooting method?

A

The starting point is at the bottom of the OSI model, the physical layer.

The examination starts from the network’s infrastructure such as cables and NICs

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5
Q

What is the Top-Down troubleshooting method?

A

The starting point is at the top of the OSI model, the application layer.

The examination starts from the application itself

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6
Q

What does documentation include?

A

Network-related notes: physical and logical topological diagrams, network and end-device backup configuration files, IP and MAC addressing schemes, and network performance baselines

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7
Q

What is the Divide & Conquer troubleshooting method?

A

Starting point is not predefined. A particular layer is selected and examined. If the layer functions, then the next layer to examine is the one above it. if the layer is faulty, then the next one should be the one below it.

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8
Q

What are the top IOS show commands?

A
  1. show interfaces
  2. show ip route
  3. show ip interface brief
  4. show running-config
  5. show startup-config
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9
Q

IOS command: show interface

A

Displays a large volume of info, including interface errors, bandwidth utilization, and interface speed

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10
Q

IOS Command: show ip route

A

Verifies network reachability. Will indicate if routing protocols are operating as they should

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11
Q

IOS Command: show ip interface brief

A

Presents only info about interfaces associated w/ the the IP protocol. Outputs a detailed summary of interface status and IP addresses

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12
Q

IOS Command: show running-config

A

Displays the complete device configuration that is being used at that time

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13
Q

IOS Command: show startup-config

A

Displays the complete device configuration that is being used when it loads

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14
Q

IOS Command: traceroute

A

Discovers the path packets take to a remote destination and where routing failures occur

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15
Q

IOS Command: ping

A

Checks network connectivity. Extended ping is able to change the source IP on the packet leaving the router

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16
Q

What is Cisco Discovery Protocol (CDP)?

A

Layer 2 (data link) proprietary protocol enabled by default on all Cisco devices. Purpose is similar to NDP.

Can be disabled on specific devices or interfaces for security reasons

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17
Q

What are CDP characteristics?

A
  1. Eavesdropping - CDP does not encrypt data so it’s vulnerable to reconnaissance type attacks which allow the attacker to map the victim’s network and more
  2. Information gathering - gathers data on local device and sends to all directly connected devices
  3. Advertisements - packets that reveal device details to neighbors also running CDP. Advertisements are stored in the neighbor table
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18
Q

What do CDP Advertisements include?

A
  • Type of device (router/switch)
  • Device name
  • Interconnected interfaces
  • IOS version
  • Next-hop IP
  • Connection’s duplex mode (full or half)
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19
Q

What is mismatch detection?

A

Feature (in CDP) that can detect a protocol mismatch.

E.g. Native VLAN mismatch and Duplex mismatch

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20
Q

What happens when a CDP mismatch occurs?

A

CDP notifies network administrator by recording the incident in a log.

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21
Q

How long is the CDP packet timer?

A

CDP sends advertisement packets every 60 seconds by default to all directly connected devices

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22
Q

IOS Command: show cdp

A

Displays global CDP info: (1) packet timer, (2) holdtime value (3) advertisements enabled/disabled

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23
Q

What is the CDP hold timer?

A

If a device does not receive CDP packets from a neighbor for 180 seconds, it will consider the neighbor dead and remove it from its neighbor table

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24
Q

IOS Command:

cdp run

A

Enables CDP on device (not port)

Recommended: Enable CDP only for port interconnections w/ other network devices

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25
IOS Command: | no cdp run
Disable CDP (on device not port)
26
IOS Command: | cdp enable
Enables CDP on specified port(s)
27
IOS Command: | no cdp enable
Disables CDP on specified port(s)
28
IOS Command: | show cdp neighbor
Displays all connected devices that CDP can communicate with
29
IOS Command: | show cdp neighbor detail
Displays more info on each device such as IP address and IOS version
30
What is Link Layer Discovery Protocol (LLDP)?
It's a Neighbor Discovery Protocol similar to CDP developed by IEE. It's a standard protocol. Not enabled by default on Cisco devices yet fully supported
31
What is the LLDP packet timer?
LLDP sends advertisement packets every 30 seconds by default to all directly connected devices
32
What is the LLDP hold timer?
120 seconds. If device does not receive LLDP packet from a neighbor in 120 seconds it will consider that neighbor dead and remove it from its neighbor table
33
IOS Command: | show lldp
Displays global LLDP info: (1) status (active/inactive) w/ packet timer and (2) hold timer
34
IOS Command: | lldp run
Enables LLDp on device
35
IOS Command: | no lldp run
Disables LLDP on device
36
IOS Command: | lldp transmit
Enables LLDP to transmit on specified port(s)
37
IOS Command: | no lldp transmit
Disables LLDP to transmit on specified port(s)
38
IOS Command: | lldp receive
Enables LLDP to receive on specified port(s)
39
IOS Command: | no lldp receive
Disables LLDP to receive on specified port(s)
40
IOS Command: | show lldp neigbor
Displays all connected devices that LLDP can communicate with
41
IOS Command: | show lldp neighbor detail
Displays more info on each device such as IP address and IOS version
42
What is a Syslog?
A standard for logging messages. Protocol is documented in RFC 5424
43
Why are System Log Events useful?
Increases a network administrator's awareness of hardware or software malfunctions, service status and security accountability. Can be used to predict component failures. Can be stored locally on the device or on a syslog server on network
44
What is Log Storage?
Log storage takes up space and can, in some cases, degrade system performance. Log message count is limited due to buffer size. RAM is erased upon device power off
45
What are Syslog Storage Methods?
1. Logging buffer 2. Console Line 3. Terminal Line 4. Syslog Server
46
What is a Logging Buffer?
A predetermined storage space allocated on RAM for syslog messages
47
What is Console Line?
A method of viewing messages locally via the console window
48
What is Terminal Line?
A method of viewing messages via a CLI program (such as Putty)
49
What is a Syslog Server?
A computer that runs syslog software. Concentrates logs from all devices on the network into once centralized location for easier mgmt and log backup. a syslog server is configured on the device, the device will send all syslog messages to the server over UDP 514 port
50
What are important pieces of the Syslog messages?
1. Message Severity | 2. Message Facility
51
What is Message Severity?
The urgency of the log message is represented by its severity level (number value 0 to 7)
52
What is a Message Facility?
The facility parameter indicates the source that generated the message (e.g. hardware, routing protocol or system service)
53
What are the Severity Levels?
``` Level 0 - Emergency (system unstable) Level 1 - Alert (immediate action required) Level 2 - Critical Level 3 - Error Level 4 - Warning Level 5* (default) - Normal but important Level 6 - Information Level 7 - Debugging ```
54
What is log message format?
1. Timestamp 2. Facility 3. Severity 4. Mnemonic (a device specific code that identifies the log message) 5. Text description
55
IOS Command: | logging host
Sets the destination syslog server
56
IOS Command: | logging trap
Determines what log severity levels to send the server
57
IOS Command: | service timestamp log datatime msec
Attaches time info to the alerts
58
IOS Command: | logging console
Shows system messages on the screen of the console connection (enabled by default)
59
IOS Command: | terminal monitor
Shows sytem message on the screen of a remote connection (Telnet, SSH)
60
IOS Command: | login buffered
Saves system messages to the RAM (enabled by default)
61
IOS Command: | logging host
Sets the destination IP of the syslog server
62
IOS Command: | service timestamps log datetime msec
Adds time records
63
What is Network Time Protocol (NTP)?
NTP is a client-server protocol responsible for the synchronization of the time and date across all devices on the network. Operates on UPD port 123 and is defined in RFC 1305
64
How do you configure time?
1. Manual configuration | 2. Automatic configuration
65
What is time Manual Configuration?
Requires manual setting on each device. Not recommended due to lack of scalability
66
What is time Automatic Configuation?
Requires an available and configured NTP server. Only the server's IP address needs to be set on the network devices. Ensures that all devices on network have accurate time/date automatically. Cisco routers can be configured to act as NTP servers
67
What is NTP Usage?
cisco routers and switches use NTP to determine although routers and switches can also act as NTP servers
68
What is Stratum Model?
Stratum levels define the distance from high-precision reference clocks which begin w/ stratum 0 to devices connected hierarchically. Servers linked to the reference clock are stratum 1, devices that receive time from servers are stratum 2, etc. Max 15 levels
69
IOS Command: | clock set
Sets clock date and time
70
IOS Command: | ntp server
Identifies NTP server's IP address
71
IOS Command: | (do) show clock
Shows time
72
IOS Command: | log timestamp
Logs device messages and events with a time and date
73
IOS Command: | ntp master <1-15>
Enables network device (router or switch) to act as an NTP server. Requires a stratum value and number of hops from an accurate time source (e.g. atomic clock)

Computer Networking (4 decks)

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